HDDs

Photo: Nick on Unsplash

In short: “Hard Disk Drives” — classic hard drives that store data magnetically on rotating metal disks.

In more detail: HDDs are considerably cheaper per gigabyte than SSDs, but, because of their mechanical construction (rotating disks, moving read head), slower, louder and more sensitive to shocks. Today they’re mainly used for large, cheap storage capacity (backups, archives), while SSDs dominate as system drives.

In Depth

How an HDD physically works

An HDD stores data by differently magnetising tiny regions of a rapidly spinning, magnetically coated metal disk (platter) — a moving read/write head floats on a wafer-thin cushion of air only a few nanometres above the surface, without touching it. This mechanical precision work is the main reason for the considerably higher access time compared to SSDs: before data can be read, the platter and head first have to move to the right position (seek time/latency), while an SSD, having no moving parts, can access any storage cell practically without delay. The platter’s rotation speed (measured in revolutions per minute, RPM — typically 5,400 or 7,200 RPM for consumer HDDs, sometimes over 10,000 RPM for server HDDs) directly affects both speed and noise.

Why HDDs remain relevant despite SSDs

Despite the speed disadvantage, HDDs remain relevant in certain use cases: at very large capacities (several terabytes, currently up to over 20 TB per drive), they’re still considerably cheaper per gigabyte than comparable SSDs, which makes them ideal for archives, backups or mass storage, where capacity per euro matters more than speed. Data centres therefore still use large HDD arrays for cold-data storage today, while data needed quickly sits on SSDs.

Disadvantages and mechanical vulnerability

Besides speed, disadvantages also include mechanical vulnerability (shocks during operation can damage the read/write head or scratch the platters, leading to permanent data loss) and audible operating noise (the spinning sound of the platters, clicking of the read/write head when positioning), which is completely absent with SSDs. Idle power consumption also tends to be higher with HDDs, since the platters have to be kept permanently spinning.

Data recovery after HDD failure

An advantage in case of data loss: a defective HDD can, under certain circumstances, be physically repaired by specialised data recovery companies, or the platters moved into an identical replacement enclosure, to still access the data — with a defective SSD, this is often considerably harder or impossible, because of the more complex, finely integrated electronics.

See also: SSDs, Hard Drives, Archives